Carbon nanotubes are the object of a considerable and constantly growing interest in the nanoelectronics sector, which still today is not yet capable of fully exploiting its exceptional features and properties, in particular in field emission devices.
Single wall carbon nanotubes and multiple wall nanotubes show an excellent electrical conductivity at room temperature.
Carbon nanotubes are different from the metal emitters in that, in the latter, the electrical resistance increases with increasing temperature, the heat increasing proportionally with the increase in the extracted current. This causes an unstable heat dispersion, which inevitably leads to the destruction of the emission source in the metal type emitters. On the other hand, however, nanotubes are very stable emitters even at very high temperatures, due to the fact that their electrical resistance decreases with temperature, thus limiting the generated heat.
In addition, other features such as the exceptionally reduced size, the ballistic conduction, the high transport mobility, and the absence of electronic migration make the carbon nanotubes ideal candidates for transistor type applications and for the interconnections in electronic devices.
Even though recently, realization and use techniques have been developed for carbon nanotubes of a certain interest, these new materials are still far from being industrially utilized, mainly due to the difficulties encountered in managing them, which may make their positioning control particularly problematic. In order to properly exploit carbon nanotube features, it is helpful to effectively control their relative position on a substrate.
The prior art, for example, provides a horizontal positioning method of nanotubes based on the nanomanipulation, which includes using an AFM rod for moving single nanotubes onto a substrate. Other gripping devices have been made that are capable of ensuring a precise control of the horizontal positioning of nanotubes, which nevertheless allow the grip from one site and the release in a final position of a single nanotube at a time.
It is understood that techniques of this type may be limited to the realization of prototypes of devices comprising horizontally-arranged nanotube arrays, since they are serial techniques which may not easily be applied on an industrial scale.
Other techniques provided in the prior art, such as ink jet printing, dielectrophoresis, and stamp printing, permit the deposition in parallel of nanotubes on a substrate, and at competitive costs.
For example, by means of the ink jet technique, a dispersion of nanotubes can be printed following preset patterns, or it can be deposited between two electrodes, with orientation and fixing of nanotubes, by means of dielectrophoresis. Moreover, nanotubes deposited on a first substrate can be taken by means of a mold of the so-called soft type and deposited by means of contact printing on a second substrate according to the pattern in relief on the mold.
The main drawback of such techniques lies in the fact that they are applicable for the realization of nanotube networks. Furthermore, the known art provides for processes in which the nanotubes are not arranged and/or handled on a substrate but directly grown, that is realized, on them.
In the process of horizontal growth of nanotubes, a silicon substrate is engraved creating grooves or trenches in which the growth of nanotubes occurs between an edge and another of the grooves or trenches. However, such processes may not guarantee a well controlled final position of the nanotubes.
On the contrary, by engraving a catalyst layer deposited on the substrate, also in this case realizing grooves or trenches, in which nanotubes are formed, better results are attained in terms of controlling the position of the nanotubes on the substrate. However, such better results may be diminished by the fact that the diameter of a nanotube depends of the width of the groove in which it grows, which in turn depends on the resolution of the patterning technique used to create the groove. The obtainment of particularly small diameters, thus nanotubes with a high potential, necessarily implies the use of extremely costly lithography techniques which, furthermore, do not guarantee a nanotube diameter smaller than a given limit (about 20 nm).
Also in order to obtain an array of nanotubes arranged vertically on a substrate, the prior art provides processes in which the nanotubes are grown directly on the substrate. In particular, nanotubes are grown on a substrate through a technique known as CVD (chemical vapour deposition). The growth process occurs by means of aggregation of carbon atoms, supplied by a high temperature hydrocarbon gas in a nanotube crystalline structure generated by a nanometric size metal particle which serves as a catalyser and crystallisation nucleus. For example, methane gas at 900° C. and at a pressure of 750 torr can be used as a source of carbon.
Though there are several procedures for obtaining catalyst nanoparticles (distributed on a substrate) on which carbon nanotubes can be grown, an effective control of the position of each nanotube is currently attained by using e-beam lithography techniques which allow localising, on the substrate, spots or dots at desired positions.
In such a manner, it is possible to obtain vertically oriented parallel nanotubes having highly controlled positions, but the entire realization procedure may be particularly slow and very costly due to the use of the e-beam lithography and, therefore, may not be acceptable in a large scale industrialisation like the one used currently for electronic production.
Additionally, there still remains however a limit to the smallest dimension of the abovementioned catalyst spots which can be obtained through e-beam lithography.
Therefore, the need to realize (on a substrate) an array of nanotubes of particularly small dimensions with a high control of their positioning, both in case of horizontal orientation and in case of vertical orientation, at a cost allowing its industrialization in the manufacture of electronic devices, has yet to be met.